This invention relates to a high-frequency power circuit for energizing at least one gas discharge lamp with two filaments, said circuit being adapted to be supplied with power from a DC voltage source, said circuit comprising semiconductor switching elements and control means for the semiconductor switching elements, as well as a high-frequency transformer having at least one primary winding which, in operation, is supplied with an AC voltage signal by the semiconductor switching elements, and having at least one secondary main winding and secondary auxiliary windings, said secondary auxiliary windings, in operation, energizing the filaments of the at least one gas discharge lamp.
Particular requirements are imposed on power circuits suitable for feeding one or more gas discharge lamps exposing an original document in a copying machine, electronic document scanner or comparable apparatus. In such circumstances, the gas discharge lamps should be capable of being switched on and off a great many times (e.g. one million times), and high standards are exacted as to the stability of the lamp luminance. It is also desirable that very quickly after transmission of a control command, e.g. after 10 to 100 milliseconds, the lamps are lit and give the desired luminance. It is also often desirable that the lamp luminance is adjustable over a relatively large range. High-frequency power circuits for these purposes are known e.g. from the publications DE 3528549, USA 4,251,752, USA 4,286,195, DE 3248017, USA 4,087,722, EP-A 0104264 and USA 3,657,598. In these known circuits, an input voltage is converted into a high-frequency voltage, which is supplied to one or more gas discharge lamps by means of one or more semiconductor switching elements, such as transistors or thyristors. Different methods are used here to limit the current through the gas discharge lamps. In some cases, the luminance of the lamps is controlled by controlling the duty cycle of the switching elements or by supplying high-frequency burst signals to the lamps. However, in none of the known circuitries, is any special attention paid to the firing process of the lamp, in particular not to the load of the filaments of the lamp during firing. Furthermore, practice has shown that the known luminance control circuitries sometimes exhibit instabilities which need not be a drawback for non-critical applications but which are intolerable for application in e.g. a copying machine.